Some microcomputer integrated circuits have a voltage controlled oscillator (VCO) on the integrated circuit. Usually the VCO is part of a phase lock loop circuit. The purpose of the phase lock loop circuit is to generate a high frequency signal that is approximately in phase with a much lower frequency crystal oscillator signal. The high frequency signal is then used to generate internal clock signals which are used to clock various circuitry on the microcomputer. The use of the phase lock loop circuit allows an inexpensive low frequency crystal (e.g. 32 KHz) to be used even when the internal clock signal frequency is much higher (e.g. 16 MHz). The phase lock loop circuit functions by phase locking the VCO output signal with the incoming crystal oscillator signal.
In some microcomputers, the VCO also serves the additional purpose of generating a free running clock to keep the microcomputer functioning when the crystal oscillator fails. Some microcomputers, such as Motorola's 68300 family of microcomputers (e.g.: XC68340, MC68332, XC68331, and XC68330 all available from Motorola, Inc. of Austin, Tex.), have a special "Limp Mode" which is used in case the crystal oscillator fails. If the crystal oscillator fails, i.e. stops oscillating, the phase lock loop circuit would normally force the VCO output signal to also stop oscillating. Unfortunately this would cause the internal clock signals to stop oscillating, and thus the microcomputer would stop functioning.
Having the microcomputer stop functioning due to a failure of the crystal oscillator is unacceptable to some microcomputer customers. Thus Limp Mode may be used by these customers to continue operation when the crystal oscillator fails. In Limp Mode, the VCO output signal is no longer phase locked to the malfunctioning crystal oscillator signal. Instead, the VCO output signal is allowed to continue oscillating as a free running clock. The term "free running" indicates that the VCO is no longer phase locked to the crystal oscillator signal. The VCO on Motorola's 68300 family of microcomputers thus has two possible modes: (1) a standard Phase Lock Loop Mode; and (2) a special Limp Mode.
The "gain curve" of a VCO is a very important characteristic of the VCO. The gain curve is a plot of the frequency of the VCO output signal (vertical axis) versus the input bias voltage (horizontal axis). Each point on the gain curve represents the frequency of the VCO (in MHz) for a given input bias voltage (in volts). The gain is also a function of temperature. Generally, as temperature decreases, the gain of the VCO increases. At low operating voltages, the slope of the gain curve will be more stable across temperature; but at higher operating voltages, the slope of the gain curve will vary significantly across temperature.
In the standard Phase Lock Loop Mode, the VCO requires a high gain in order to produce a broad range of VCO output frequencies across temperature, operating voltage, and manufacturing variables, as required by some customer applications. In the special Limp Mode, the VCO requires a lower gain in order to prevent the free running VCO output signal from oscillating at frequencies beyond the maximum operating frequency of the microcomputer.
If the gain of the VCO is too high in special Limp Mode, the VCO output signal may be an excessively high frequency. This excessively high frequency VCO output signal will then be used to generate excessively high frequency internal clock signals. These internal clock signals may be too high of a frequency to properly clock the various circuitry on the microcomputer. Consequently the microcomputer may not function properly if the gain of the VCO is too high in special Limp Mode.
Thus, in the standard Phase Lock Loop Mode, it is desirable for the VCO to have a high gain; and in the special Limp Mode, it is desirable, and in fact necessary, that the VCO have a sufficiently low gain. The previous VCOs on Motorola's 68300 family of microcomputers treated this problem as a trade-off. The gain of the previous VCOs was selected to be somewhere between the desired high gain for Phase Lock Loop Mode and the required lower gain for special Limp Mode.
Under normal operating conditions and temperatures, this trade-off solution was adequate. Unfortunately, however, this trade-off solution was not adequate under the extreme conditions of high voltage and low temperature. At the highest voltage and the lowest temperature, the VCO gain was too high in special Limp Mode and the microcomputer sometimes did not function properly. Also, this trade-off solution resulted in VCO output signal frequencies which covered an excessively broad range.
Because microcomputers may be used in rugged applications, such as automobile engines, most microcomputers must be able to operate across a wide range of temperatures and voltages. For example, Motorola's 68300 family of microcomputers must presently be able to operate across a voltage range of 3.0 to 5.5 volts and across a temperature range of -55 to 125 degrees Celsius. Thus, a solution to the VCO gain problem was needed that would work across the entire specified ranges of temperature and voltage.